The generation of chlorous acid by the acidification of an aqueous chlorite salt solution or stabilized aqueous chlorine dioxide solution (stabilized chlorite salt solution) by an acid is well known by the following reaction:Na+ClO2−+H+→H+ClO2−+Na+  (1)
It is also well known that over time, chlorous acid slowly decomposes to chlorine dioxide by the following reaction:5HClO2→4ClO2+HCl+2H2O  (2)This reaction (2) predominates at low acid and high chlorite concentrations, making the reaction difficult to control, especially in high alkalinity water supplies. Further, this decomposition is slow. At chlorite concentrations greater than 20,000 mg/L, the reaction rate is 5 minutes at a pH<0.5. However, if the pH of the same chlorite solution is increased to >1.0, the reaction is not complete after 60 minutes.
It is also well known that in an oxidizing environment, such as in the presence of chlorine or an anode, chlorine dioxide can be generated from chlorous acid by the following reaction:HClO2−e−→ClO2+H+  (3)
It is further known that chlorous acid is generated by the acidification of chlorate salt by the following two-step reaction:Na+ClO3−+H+→H+ClO3−+Na+  (4)HClO3+HCl→HClO2+HOCl  (5)In this reaction, hypochlorous acid, the ionized form of chlorine in water, is also generated reaction (5). The generation of chlorine dioxide occurs via the one of the following mechanisms:HClO3+HClO2→2ClO2+H2O  (6)+HOCl+HCl→Cl2+H2O  (7)or2HClO2+HOCl+HCl→2ClO2+2HCl+H2O  (8)
The generation of chlorine dioxide from chlorate salt, however, is very difficult to control. In practice, excess acidity is required to start the reaction, but if the acidity is too high, the following side reaction predominates, and little to no chlorine dioxide is generated.ClO3−+6H++6e−→Cl−+3H2O  (9)
In practice, the following reduction reactions are used to generate chlorine dioxide from chlorate salt. High concentrations of all precursors must be used to start the reactions, but when the reactions do not go to completion, undesirable byproducts or unreacted precursor materials contaminate the chlorine dioxide solutions. In addition, the chloride ion must be present, either from the decomposition of chlorate or the addition of the chloride ion itself, for chlorine dioxide to be generated. Overall reactions for the generation of chlorine dioxide from chlorate are listed below.2ClO3−+SO2→2ClO2+SO42−  (10)4ClO3−+CH3OH+4H+→4ClO2+HCOOH+3H2O  (11)ClO3−+Cl−+2H+→ClO2+Cl2+H2O  (12)2ClO3−+H2O2+2H+→2ClO2+O2+2H2O  (13)
It is further known that a mixture of chlorite salt and chlorate salt in the presence of hydrogen ion will generate chlorine dioxide by the following overall reaction:2H++ClO2−+ClO3−→2ClO2+H2O  (14)This reaction is also shown in a different format in reaction (6). However, the rate of reaction to chlorine dioxide of the chlorite salt is 100 times faster than the rate of reaction to chlorine dioxide of the chlorate salt.
The use of chlorine dioxide in many applications has been limited due to the inability to control the reaction chemistries and the inefficiency of the reactions in solutions. Since chlorine dioxide is an unstable gas, even in solution, it must be generated on-site and used shortly after generation. Large-volume industrial applications such as pulp and paper bleaching, municipal water pretreatment and disinfection, food processing disinfection, and cooling tower disinfection have been successful due to the ability in these applications to safely handle concentrated and aggressive oxidizers and acids.
Some consumer and medical applications have also had success. Chlorite salt-based toothpastes, mouthwashes, and disinfecting skin gels use either the pH of the mouth or a weak-acid activator to slightly acidify the chlorite salt so that some chlorous acid is formed. The chlorous acid will then slowly decompose to chlorine dioxide by reaction (2).
It is also described in U.S. Pat. No. 6,200,557 B1 that in a topical solution designed to treat HIV, the chemical addition of phosphates will retard the loss of chlorine dioxide from solution at pH 6–7.4. This is especially beneficial in this topical application so that contact time of chlorine dioxide on the skin surface is increased to allow better treatment of the HIV.
However, in all of the prior art processes, controlling the reactions has remained a major obstacle. In addition, unreacted precursor components and reaction by-products are undesirably carried over into the product solutions. Also, in many instances, the pH of the product is so low due to the excess acid in solution that it cannot be used in certain applications.